The present invention relates generally to implantable cardiac therapy devices, and more particularly, to an active overload detection and protection circuit for such devices.
Implantable cardiac therapy devices include implantable pacemakers and implantable cardioverter-defibrillators. An implantable pacemaker monitors the intrinsic electrical activity of the patient's heart and if a natural heart beat is not detected within a prescribed time period, the pacemaker delivers (via a lead system) an electrical stimulation or pacing pulse to force the heart muscle tissue to contract, thereby assuring that a minimum heart rate is maintained. In this way, bradycardia is terminated or prevented. Contemporary implantable cardioverter-defibriliators (ICDs) monitor the intrinsic electrical activity of the patient's heart in accordance with a diagnostic or detection algorithm by analyzing electrograms (EGMs) generated by sensing electrodes positioned proximate the sino-atrial and/or atrio-ventricular node of the patient's heart, and most advantageously, in the right ventricular apex of the patient's heart.
Typical current-generation ICDs are capable of delivering various types or levels of cardiac therapy (i.e., "tiered therapy"). The first type or level of therapy is bradycardia and antitachycardia pacing (ATP), in which a low level of electrical energy (generally between millionths to thousandths of a joule) is delivered to the patient's heart (via a lead system) in order to correct detected episodes of bradycardia or tachycardia, respectively. The second type or level of therapy is cardioversion, in which an intermediate level of electrical energy (generally between 1-5 joules) is delivered to the patient's heart (via a lead system) to terminate a detected episode of ventricular arrhythmia (e.g., a detected heart beat in the range of 130-190 beats/minute) or an ongoing episode of tachycardia that ATP therapy has failed to tenninate. The third type or level of therapy is defibrillation, in which a high level of electrical energy (generally above 15 joules) is delivered to the patient's heart (via a lead system) in order to terminate a detected episode of ventricular fibrillation or an episode of ventricular tachycardia which has degraded into ventricular fibrillation due to failure of cardioversion therapy. The defibrillation energy is typically stored in a defibrillation energy storage capacitor ("output capacitor") which is charged by a high-voltage charging circuit, and then delivered as an electrical shock(s) by means of a high-voltage output switching circuit which discharges the output capacitor.
At various times during normal operation of an implantable cardiac therapy device, the device circuitry is particularly susceptible to being damaged by external high voltage transients, such as those produced when the patient is subjected to external defibrillation. Such external high voltage transients can induce large (&gt;150 mA) and slow (longer than 10 ns) current pulses within the device circuitry, which can cause significant damage to the device circuitry. Presently available ICDs include passive protection circuitry to prevent such current overload conditions from arising and damaging the device. Since this protection circuitry is passive, it does not have the capability of actively detecting a current overload condition. Further, the passive current overload protection circuitry is inadequate to protect the device circuitry against certain current overload conditions, such as those occasioned by external high voltage transients encountered during external defibrillation, especially during periods when the device is particularly vulnerable to damage, e.g., during pacing, plethesmography, and output capacitor discharge periods.
Based on the above, it can be appreciated that there presently exists a need in the art for an implantable cardiac therapy device which overcomes the above-described disadvantages and shortcomings of the presently available devices. More particularly, there presently exists a need in the art for an implantable cardiac therapy device which includes an active overload detection and protection circuit which is capable of actively detecting a current overload condition and of placing the device in a current overload protection mode which minimizes potential damage to the device circuitry. The present invention fulfills this need in the art.